1. Field of the Invention
This invention generally relates to a sprocket for a bicycle. More specifically, the present invention relates a top sprocket for a rear sprocket assembly and a rear sprocket assembly for a bicycle, which has a simplified mounting structure and provides smooth reliable shifting.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle as well as the frame of the bicycle. One component that has been extensively redesigned over the past years is the bicycle drive train. Specifically, manufacturers of bicycle components have been continually improving shifting performance of the various shifting components such as the shifter, the shift cable, the derailleur, the chain and the sprocket or sprockets.
One particular component of the drive train that has been extensively redesigned in the past years is the sprocket assembly for the bicycle. More specifically, the bicycle sprocket assembly has been designed with improved sprockets to provide smoother shifting. More specifically, the Hyper Glide sprocket assembly (HG rear sprocket assembly) described in U.S. Pat. No. 4,889,521 to Nagano has been developed and marketed worldwide. This sprocket assembly has been well accepted in the market and this engineering design for the sprocket assembly has become one of the industrial standards. Specifically, the HG rear sprocket assembly used with derailleurs in racing and mountain bicycles has become an industrial standard.
The HG sprockets typically have a chain guide surface on the smaller sprocket side of each sprocket, except for the smallest sprocket. The chain guide surface is positioned to receive a link plate and pin of the chain so as to move the chain much closer for smooth down shifting. Down shifting typically means shifting from a smaller rear sprocket to a larger rear sprocket so as to decrease gear ratio. A pair of adjacent sprockets are positioned relative to each other and relative to the chain guide surface with a particular phase relation. In order to maintain such phase relation, each sprocket has splines formed at its inner periphery. One of the splines is a different shape from the other splines to guide the sprockets on mating splines formed on the outer body of a multiple freewheel or free hub. The different shaped splines insure that adjacent sprockets are oriented in the proper phase relation.
Typically, in this type of sprocket assembly, the smallest top sprocket also has splines formed on its inner periphery, and a lock ring is used to fix the smallest sprocket to the multiple freewheel or free hub. In this construction, all of the sprockets from smallest to largest have inner splines. While such a sprocket design provides for smooth reliable shifting, this construction can be relatively complicated and expensive to manufacture and assemble.
Accordingly, in recent years, the smallest sprocket has been formed with screw threads as a conventional sprocket, while the remaining sprockets are formed with inner splines. In this construction, the lock ring is eliminated to achieve a sprocket assembly that is easier and less expensive to manufacture and assemble. However, in such a sprocket assembly, because the smallest sprocket is threaded onto the outer body, an unstable phase relation with the second sprocket occurs. This unstable phase relation causes unstable down shifting from the smallest sprocket to the second sprocket. Accordingly, there is a strong demand to improve shifting from the smallest sprocket to the second sprocket on such a sprocket assembly without significantly increasing the complexity in manufacturing and assembling the sprocket assembly, and thus, increasing the cost of the sprocket assembly.
In view of the above, there exists a need for a top sprocket for a rear sprocket assembly and a rear sprocket assembly for a bicycle which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.